The present invention relates to a dual-slope analog to digital converter, and more particularly, to a dual-slope analog to digital converter which is time independent.
Prior art dual-slope analog to digital converters are based upon the principle that the slope of a linear integrating element is proportional to the magnitude of the signal being integrated. In a typical integrator of this type, an unknown analog voltage is applied to a linear integrator (such as an operational amplifier with a capacitor in its feed back circuit) and integrated for a predetermined time period. At the end of this first integration period, the output of the integrator will have increased at a constant slope from a first to a second value. At the completion of the first integration period, a signal of a known value and opposite polarity to the first, unknown, signal is applied to the integrator causing a negative slope integration of the known signal. A digital measurement is then taken of the time required for the output of the integrator to decrease from the second to the first value (the value at the initiation of the conversion process). This measurement is normally taken by applying a pulse train having a predetermined frequency to a digital counter for the duration of the second integration period. The count in the counter at the end of this period is indicative of the magnitude of the analog signal to be converted. Analog to digital converters of the foregoing type are exemplified by U.S. Pat. No. 3,826,983.
The primary drawback of the prior art converters is that they are time dependent. Accordingly, any fluctuations in the frequency of the pulse train applied to the counter or in the length of the first predetermined integration period will result in an incorrect measurement.